Electron emission material



Patented July '5, 1932 UNITED (STATES PATENT OFFICE MARK N. FREDENBURGH, OF EAST ORANGE, NEW JERSEY, ASSIGNOR TO WESTING- HOUSE LAMP COMPANY, A CORPORATION OF PENNSYLVANIA ELECTRON EMISSION MATERIAL Application filed July 18, 1928.

This invention relates to electron discharge devices employing electron emitting hot cathodes, and more particularly relates to a method of producing the coating material employed in electron emitting hot cathodes of the oxide coated type and'comprises essentially in a method ofproducing intimately admixed barium and strontium carbonates in a preferred physical form suitable for use as the coating material of said cathodes. This invention is essentially an improvement in the process as described and disclosed by copending application, Serial No. 587,270, filed Sept. 11. 1922, by Duncan MacRac, entitled Electron emitting device and method of manufacture, which copending application is assigned to the Vestinghouse Lamp Company, the same assignee of the present invention.

In the preparation of cathodes for electron discharge devices, one of the common sources of electron emission is from the heated oxides of the alkaline earth metals. barium, strontium and calcium, either singly or mixed in various proportions. Since the oxides of the alkaline earth metals are chemically active and readily absorb atmospheric impurities of moisture and carbon oxides, etc., the process of applying them to a metal base material for use as a hot cathode is difficult. In place of direct application of the alkaline earth oxides tothe base metal of a hot cathode, it is customary to apply instead, stable compounds of they alkaline earth metals, such as the carbonates as by the above mentioned process, which may be subsequently decomposed or dissociated to the oxide when heated in a vacuo. 1

It hasbeen customary heretofore to employ as a coating material. the ordinary commereial grades of alkalineearth metal carbonates, which hare been purchasahle upon the market.

This. carbonate material has been finely ground or comminuted and the, desired admixture of the barium, strontium or calcium Serial No. 293,801.

rial is then applied to the surface of the cathode in any suitable manner. The moist coating is rendered substantially anhydrous by heating to approximately 700 C. to 900 C. in an atmosphere of CO Successive coatings in an analogous manner may be applied if desired, or successive alternate coatings of the alkaline earth metal carbonates may be so applied. A suspension of mechanically admixed finely comminuted barium and strontium carbonates, in approximately equal proportions, is the composition usually employed.

Whereas the process of coating as disclosed in above mentioned copending. application, Serial No. 587,270, and the results obtained thereby when the coated cathodes are employed in electron discharge devices, have been highly satisfactory, it has been found that the degre of dispersion or the intimacy of the admixture of the two alkaline earth metal carbonates has a pronounced effect upon the operating efiiciency of the hot cathode. It has also been found that the degree of purity, particle size and degree of particle size difference, density, relative proportions of components, etc., all exert an effect upon the life, emission qualities and operating efllciency of the hot cathode and electron discharge device of which it is an incorporated art.

It has also been determined that the physical state of the coating material controls in a marked degree the physical characteristics of the resultant coated cathode. With heavy dense carbonate material of the physical characteristics as brought out in my copendknown process.

It is one of the objects of the present invention to provide a coating material com- Silt v creased wit prised of barium and strontium carbonates of high absorptive roperties towards the fluid suspension me ium, which is substantially pure, intimately admixed, of a predetermined uniform particle size and particle size difference and which occupies a maxi mum volume per unit weight of material when applied to the surface of hot cathode materia Another object of this invention is to provide a process which would be operable on a commercial scale to produce such a product consistently.

Another object of this invention is to produce material for use as a coating for hot cathodes which cathodes would have uniform operating characteristics when the same are incorporated in an'electron discharge device.

Other objects and advantages will become apparent as the process is more fully disclosed.

Broadly, my invention contemplates producing a substantially pure intimate admixture of barium and strontium carbonates by simultaneous precipitation from aqueous solution under such conditions that the particles of the precipitated product are substantially uniform in physical characteristics, readily filterable and which would possess a high absorptive propert towards the fiuid suspension medium of t e coating composition which property is characteristically indecreased density of the preci itated carbonate material.

n employing hot cathodes of the oxide coated type it is commonly considered that the barium component thereof is the activat mg constituent and it is desirable from most standpoints to employ this component in amounts u to 50 er cent. The strontium carbonate 1s consi ered to be the inert or carrier component of the barium.

Whereas heretofore the rior art has employed such admixtures of arium and strontium carbonates in varying amounts and proportions up to approximately equal proportions, it has been impossible to control within narrow limits by mechanical admixture methods the physical characteristics of the coating material, the depth or thickness of the coating or the operating characteristics glf o the filamentary cathode prepared there- While I do not wish to be bound by any particular theory it must be appreciated that:

in the type of composite filamentary cathodes as contemplated within the scope of the'gresent invention the de th of the'coating t ereon controls in a,.wi e measure the operating characteristics of the composite cathode.

The metal portion of thecom osite cathode serves as the heater element to eat the coat ing'to the operating temperature. In a thin dense coating the operation temperature may be obtained more uickly than in a thick coating but is more su ject to temperature variations that may be due to voltage fluctuations .in the filamentary current supply. A thicker coating appears to be more reliable in operating characteristics under such current fluctuations or when the cathode is heated directly or indirectly from an alternating current source. Moreover, a dense thin coating may offer mechanical diiiiculties in exhaust and activating procedures, etc., not normally encountered in thicker less dense coatings. Moreover, it is extremely desirable from most standpoints to maintain the coating composition constant with respect to this factor between respective cathodes.

One of the? common methods for the production of alkaline earth metal carbonates is to precipitate an aqueous solution with carbonic acid or alkaline carbonates. This general method of production is old in the arts but I have made certain improvements therein as Will be more fully disclosed.

Heretofore it has been customary to eiiect the precipitation of barium and strontium carbonates by this general process without regard to the physical constants durin the precipitating reaction. As above note the physical characteristics of the coating material have a marked effect upon the operating eificiency of the hot cathode made therefrom. I have determined that the physical nature of the recipitate varies markedly with variation 1n the hysical constants of the precipitating reaction, such as temperaspeed of recipitating reaction, and the particular al aline earth metal compound emplo ed in solution.

he usual alkaline earth metal carbonate precipitate produced by the above mentioned general method of precipitation is diilicult to lter. The exact reason therefore is not apparent but the precipitate tends to clog up a when precipitated by the above general method under practically thesame physical com stants ofprecipitation and that the respec tive-reaction impurities of nitrate, chloride.

hydroxide of the precipitant are variously included within the precipitated carbonate material. For this reason I prefer to em,- ploy the hydroxide compound although the other compounds may be employed provided the physical constants of the recipitating. reaction be altered to conform t erewith and to produce the same result that I obtain by the process -I will hereinafter more fully describe.

I have determined that the temperature of precipitation of the carbonates effects in a marked degree the crystal structure of the barium and strontium carbonates, especially when the same are simultaneously precipitated from an admixed solution of the hydroxides. At temperatures approximating 50 C. I am enabled to obtain a precipitate of an entirely different physical nature than what heretofore has been obtained and'the same may be easily and readily separated by filtration from the mother liquor.

At other temperatures, above or below this determined temperature the physical structure of the precipitate reverts again to'the diflicultly filterable product. The exact cause therefore is not at present known. The phenomena may be due to the presence of water of hydration at all other temperatures or it may be due to some definite crystal structure change in the precipitate. The fine subdivision of particles makes it difiicult to accurately determine this factor.

Whereas this phenomena has been detected specifically with respect to the solution of the alkaline earth metal hydroxides it is not construed thereby to be so restricted as it is obvious that the solutions of the nitrate or the chloride for instance would act in an analogous manner and would require only an adjusting or alteration in the physical constants of the precipitating reaction to bring about the desired results.

In addition thereto I have determined that by regulating the time factor during the precipitating reaction I may produce a relatively uniformly grained carbonate product which yields the maximum apparent volume 1) per unit weight material permissible to obtain from this precipitate. This type of material is essential for the purpose of obtaining a dimensionally thick coating of activatin material on a hot cathode for the same unit weight material employed thereon.

I have also determined that I can assure the absolute elimination of deleterious adsorbed or absorbed impurities by utilizing calcined barium and strontium oxides in the forming of my-solution rather than the rela tively impure products obtained on the market. By calcining I insure the conversion to oxide or water insoluble compounds of all or substantially all of the heavy metal impurities. Thereafter the calcined material may be slacked in the usual manner and the water solution of the hydroxides filtered free of insoluble matter and made up to any desired stren h or concentration. V

I have a 0 determined that in addition to the calcinin of the alkaline earth metal the use of a car onate precipitant which is substantially completely decomposable, dissociable and volatile by heat is essential. Carbonic acid gas does not substantially effect the desired result as it is difiicult toreduce the time interval of the precipitating reaction to the desired minimum. I therefore prefer to employ an aqueous solution of normal ammonium carbonate substantially free of bicarbonates or carbamates. The formation of this solution will be hereinafter described.

The ammonium reaction products when this precipitant is employed are all-volatile at relatively low temperatures and are substantially removed by heat from the precipitate if occluded therein.

. I have also determined that the factor of dilution or ercent total solids in solution at the time o precipitation is a factor directly effecting the purity of the product or as may be better known as the amount of occluded impurities.

As a specific embodiment of my invention 1 disclosed.

I first produce a solution of barium and strontium hydroxides having an approximate dilution or concentration of total solids of 570, as follows:

Barium hydroxide C. P. 811 0) 80 grams Strontium hydroxl e (C. P. SE 0) 90 grams istllled water 3300 m.l.

water. Any ammonia compounds retained in the precipitate'are volatile and decomposable when heated. ==This solution is prepared as follows:

Ammonium carbonate (C. P. lump) 1100 gins. Ammonium hydroxide (C. P. so. gr. .90) 470 m.l. Distilled water 1987 m.l.

The ammonium hydroxide is added to the 6 heated to and maintained at approximately C., with occasional stirring, until solution is complete. The solution is then filtered twice by gravity throu h a No. 14 Whatman filter paper. The specific gravity of this solution should be approximately 1.130 at 25 C. This solution I will referto as Solution II.

In preparing the admixed barium and strontium carbonates by my process, I first heat Solution 1, contained in a pyrex glass beaker of 4 liters capacity, to a temperature of C. To this solution I add 150 111. l. of Solution 11', which has been preheated to a temperature of 50 C., slowly and at a constant rate so that the approximate time of precipitation is one minute. During the precipitation of the carbonates I maintain vigorous stirring.

Following precipitation I permit the carbonates to settle, wash by decantation with hot water and then vacuum filter and wish twice with hot water on a Buchner funnel fitted with a No. 50 Whatman filter paper.

The material is then placed in a pyrex glass tray and dried at 110 C., and is ready to be used in the coating process, as described in the above mentioned copending application by Duncan MacRae, Serial No. 587,270 by milling to intimately admix with the suspending li uid.

One of t e advantages in using this material in the coatingcomposition is that as prepared and dried it is ready for use without further comminuting or admixture with other alkaline earth metal carbonates. also obtain by simultaneous precipitation a degree of dispersion or intimate admixture of the barium constituent therein, which has heretofore been unobtainable. By controlling the temperature of precipitation, time of precipitation and concentration of solution, a preferred particle size and physical structure is imparted to the precipitate which is consistent from batch to batch. This physical structure and uniformity in particle size, density or absorptive property of the precipitate, is essential in order to obtain a uniformity in the rocedure of applying the material to the fi amentary cathode. Furthermore, the coating obtained through the use of this material is relatively thick as regards usual coatings heretofore obtained and-exhibits a uniformity in operating characteristics when utilized in an electron dischargedevice not heretofore obtained.

As an indication of the differences in density of precipitated carbonates as may be obtained through the practice of this invention as compared .with the product I obtain from the practice of the process described 5 in my copending application Serial No.-292,-

. m /cc) 037, above identified, the weight in grams per cubic inch of the dried material by the atter process is approximately 16.0 grams (dens.=.969 mg/cc) whereas the weight of the dried product by this process is approximately 7.0 grams per cubic inch (dens.=,.409 B These weights will vary somewhat from lot to lot depending upon the precipitating conditions, etc., but the densities of the materials produced by the two processes will remain approximately in the above mentioned respective proportions.

In applying the material to a filamentary cathode the continuous coating method as described by Duncan Macltae is em loyed. Whereas the material as prepared y the process as described in my copending application Serial No. 292.037 entitled Electron emission material and method of production is dense and requires but four coatings to apply to the surface of the filamentary cathode the requisite weight desired (approximately 4 miligrams per sq. cm. surface area), the material prepared by this process is relatively light and the coating processrequires approximately ten operations to apply an equivalent weight of material upon the filament surface. Moreover, where the material of the copending application forms a closely adherent dense coating, the material of this application forms a loosely adherent heavy thick coating, due to the wide difference in the relative densities of the two materials.

As above stated for some purposes the type of oxide coated cathode prepared from this material is desirable and such cathodes will be found to activate more readily, that is, be more readily converted to oxide, and to yield more uniform manufacturing and operating results than heretofore obtainable by prior production methods.

Having broadly outlined the scope of my invention and specifically described a process for the production of a definite admixture of barium and strontium carbonates suitable for I use as coating material for a specific type of oxide coated hot cathodes, it is apparent that. there are many variations which may be made in the procedure described or in the specific carbonate proportions mentioned therein without departing from the nature of my invention. Such departures'and variations are contemplated as may fall within the following claims.

What is claimed is:

1. The method of producing barium and strontium carbonates in a preferred filterable crystalline form which comprises precipitating a solution containing barium and strontium with normal ammonium carbonate preclpitant at temperatures approximating 50 C. V

2. The method of forming barium and strontium carbonates in a preferred filterable crystalline form having a minimum particle size contrast which comprises precipitating a solution containing barium and strontium with a solution of normal ammonium carbonate precigitan't at temperatures a proximating 50 maintaining during t e precipitatlng reaction a mmlmum time interval.

3. The method of forming intimately ad mixed barium and strontium carbonates which comprises forming a water solution of substantially pure barium and strontium hydroxides in any desired admixed proportion, simultaneously precipitatin the alkaline earth metal carbonates there rom by adding thereto a solution of normal ammonium carbonzte, while maintaining throughout the precipitating reaction a temperature approximatin 50 C.

4. 'fiie method of forming intimately admixed barium and strontmm carbonates which consists in forming a solution containing substantially ure rium and strontium hydroxides in t e desired. proportion, heating said solution to approximatel 50 0., precipitating the heated solution wit normal ammonium carbonate in a short time interval while maintaining the temperature of the solution at 50 (3., and thereafter filtering and drying the precipitated carbonates.

5. The process of ormmg an intimate admixture of alkaline earth metal carbonates, which consists in forming an aqueous solution containing barium and strontium hydroxides in approximate equal amounts, precipitatin said alkaline earth metals therefrom as t e carbonate by the addition thereto of a solution of normal ammonium carbonate, in a minimum time interval to reduce the particle size contrast thereof while maintaining the temperature of the solution at all times durlng the precipitating reaction at approximately 50 0., and thereafter washing and collecting the precipitate in any suitable manner.

6. As an article of manufacture, an admixture of barium and strontium carbonates having a uniform particle size and having a density approximating .40 grams per cubic centimeter.

In testimony whereof, I have hereunto sub-' scribed my name this 17th da of July 1928.

MARK N. FRED NBURGH. 

